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Examining a typical pressure-enthalpy chart, it is generally believed that the evaporator benefits from a complete stream of liquid entering the coil. The conclusion from this is that maximum enthalpic capacity is achieved by subcooling or even super-subcooling the entering liquid and that anything less than a full column of liquid might limit the amount of heat transfer that the evaporator can accomplish. The evolution in refrigerant flow testing and mapping is now providing support for a different view. This paper will examine the net effect of an altered bi-phase flow (ABF) regime in evaporative coils, with consideration of a high vapor fraction and turbulent refrigerant flow (HVFT). Furthermore, the pressure-enthalpy chart lends improving evaporator pressures, control during transient operation, refrigerant density at the compressor. and rate of heat transfer among other factors can combine to reduce energy consumption. Two test and verification projects in supermarket medium- and low-temperature display cases and storage applications provide significant support for examining old rules-of-thumb. The first project involves a deli service case tested under laboratory conditions. This test demonstrates the effect of evaporator operation utilizing an ABF regime and compares this to the operation of a pulse-type electronic expansion valve system. The results demonstrated operation with the ABF regime permitted increased compressor suction pressure, improved product temperature, provided more stable refrigerant temperatures, and improved case humidity. The second test and verification project involves field retrofitting of existing direct expansion evaporators to the ABF regime and measuring the results. The resulting change in refrigerant flow regime demonstrated improved performance, resulting in consistent and reduced conditioned supply air and product temperatures, improved oil return, reduced compressor discharge temperatures, and increased evaporator pressure. Product quality and energy savings were also measurable. [ABSTRACT FROM AUTHOR]

The most common problem affecting residential and light commercial heating, ventilating, and air-conditioning (HVAC) systems is slow refrigerant leaks. Equipment users are usually not aware of a problem until most of the refrigerant has escaped. The concept of a nonintrusive refrigerant charge indicator was developed. The sensor, based on temperature measurements, appears to be inexpensive and easy to incorporate into existing heat pumps and air conditioners. The design of the refrigerant charge indicator is based on the fact that when refrigerant starts to leak, the evaporator coil temperature starts to drop and the level of liquid subcooling drops. When the coil temperature or liquid subcooling drops below a preset reading, a signal can be activated to warn the equipment user that the system is undercharged. A further drop of coil temperature or liquid subcooling below another preset reading would trigger a second warning signal, such as a red warning light, to warn the equipment user that the unit now detects a leak and immediate action should be taken. The warning light cannot be turned off until it is re-set by a refrigeration repairman. Temperature sensors are becoming very accurate and low in cost compared with pressure sensors. Using temperature sensors to detect refrigerant charge status is inherently nonintrusive, inexpensive, and accurate. This paper provides laboratory test data on the change of indoor coil refrigerant temperature and subcooling as a function of refrigerant charge for a 2-ton off-the-shelf air conditioner and a 2-ton split heat pump system. The data can be used in designing the indicators for refrigerant loss. [ABSTRACT FROM AUTHOR]